1. Field of the Invention
The invention relates to a signal processing device, and more particularly, to a signal processing device for processing a passband signal.
2. Description of the Prior Art
Digital data transmission becomes important day by day in modern electronic communication systems. In conventional digital data transmission systems, digital transmission signals generally include a series of encoded symbols, and each of them represents a specific quantity of data bits in the digital transmission signals. Quadrature amplitude modulation (QAM) is a typical encoding method, wherein several continuous bits (e.g. 6 bits or 7 bits) are encoded into corresponding encoded symbols, and each of the symbols is represented in form of a complex signal, including an in-phase component I and a quadrature-phase component Q. The value of the complex signal is located on one of the predetermined points (a.k.a. constellation) in a specific number on a complex plane. The complex signal is then modulated on a radio frequency (RF) carrier. There are still several types of modulation, such as vestigial sideband modulation (VSBM), staggered QAM, etc.
In a typical communication system, since the characters of the communication channel are not always perfect (such as multi-path), an equalizer is required for a receiving end to resolve inter-symbol interference (ISI) problem. However, a typical equalizer requires a training sequence to train the equalizer before the communication is started. In such a manner, although inter-symbol interference (ISI) problem is resolved, after the training sequence is applied, the throughput of the system decreases accordingly. Therefore, various types of blind equalization (i.e. a training method without any advance training sequence) have been introduced. Besides, the ISI problem is also due to “carrier not synchronization” between a transmitter and a receiver. The blind equalization can be divided into passband blind equalization (cf. U.S. Pat. No. 5,799,037) and baseband blind equalization (cf. U.S. Pat. No. 5,754,591). In a circuit, baseband blind equalization is easier to implement than passband blind equalization, but due to the interaction between the equalizer and a carrier synchronization circuit, there is a serious problem on convergence. Therefore, various types of passband blind equalization are introduced to solve the problem on convergence.
Please refer to FIG. 1 showing a conventional signal processing device 100. The signal processing device 100 is for processing a passband signal (PBS) to generate an equalized signal (ES), which includes a feedforward equalizer (FFE) 110 for generating a first passband signal (PBS1) according to the PBS, a feedback equalizer (FBE) 140 for generating a second passband signal (PBS2) according to a sliced signal (SS), an adder 120 coupled with the FFE 110 and the FBE 140 for generating the ES according to the PBS1 and the PBS2, and a quantizer 130 coupled with the adder 120 for generating the SS according to the ES. The quantizer 130 functions similarly to a data slicer in that it can slice the data of the ES in a specific slice mode (i.e. using a specific threshold).
In the structure shown in FIG. 1, the quantizer 130 takes the SS as a correct signal and feeds it back to the FBE 140 in order to eliminate the interference between symbols of a post-cursor. When the SS generated by the quantizer 130 is incorrect, the incorrect signal is still fed back to the FBE 140 so that the interference between symbols is not eliminated but instead may become more severe, resulting in the system being unable to converge. This is a main problem of the conventional art.